Millikan, Robert Andrews

MILLIKAN, ROBERT ANDREWS

(b. Morrison, Illinois, 22 March 1868; d. Pasadena, California, 19 December 1953)

physics.

Millikan was the son of Silas Franklin Millikan, a Congregational preacher, and Mary Jane Andrews, a graduate of Oberlin who had been dean of women at a small college in Michigan. Raised in Maquoketa, Iowa, where his family moved in 1875, young Millikan enjoyed a storybook Midwestern American boyhood, fishing, farming, fooling, and learning next to nothing about science. In 1886 he enrolled in the preparatory department of Oberlin College and, in 1887, in the classical course of the college itself. Mainly because he did quite well in Greek, at the end of his sophomore year he was asked to teach an introductory physics class. Glad to have the job, Millikan plunged into the subject, liked it, and soon decided to make it his career.

Millikan graduated from Oberlin in 1891 and continued to teach physics to the preparatory students while successfully pursuing a course of self-instruction in Silvanus P. Thomson’s Dynamic Electric Machinery. Awarded an M.A. for this achievement, in 1893 Millikan entered Columbia University on a fellowship as the sole graduate student in physics. He was impressed by the lectures of Michael I. Pupin, who emphasized the importance of mathematical techniques, and by the experimental deftness of Michelson, under whom he studied at the University of Chicago in the summer of 1894. Receiving his Ph.D. in 1895, Millikan went to Europe for postgraduate study, financed by a loan from Pupin. He heard Poincaré lecture at Paris, took a course from Planck at Berlin, and did research with Nernst at Göttingen. In 1896, the excitement of the discovery of X rays still fresh in his mind, Millikan joined the faculty of the University of Chicago as an assistant in physics.

There he soon met Greta Irvin Blanchard, the daughter of a successful manufacturer from Oak Park, Illinois. By the time the young couple was married in 1902, Millikan was pouring a large fraction of his considerable energies into the development of the physics curriculum, especially the introductory courses. In conjunction with this work, he wrote or coauthored a variety of textbooks and laboratory manuals which, like his First Course in Physics (1906), written with Henry Gale, quickly became standards and sold steadily through the years. In 1907, largely because of his outstanding pedagogical achievements, Millikan was promoted to an associate professorship.

But Millikan was acutely aware that at the University of Chicago the major rewards went to those who contributed to the advancement of knowledge. Although he had consistently done research, even his most recent investigation, on the photoelectric effect, had failed to yield significant results. Unaware of Einstein’s explanation of the effect, Millikan used a spark source of ultraviolet light to determine conclusively whether the photocurrent from various metals varied with temperature; as he found, it did not. Eager to earn a reputation in research, about 1908 he decided to shelve the writing of textbooks and concentrate on his work in the laboratory.

By 1909 Millikan was deeply involved in an attempt to measure the electronic charge. No one had yet obtained a reliable value for this fundamental constant, and some antiatomistic Continental physicists were insisting that it was not the constant of a unique particle but a statistical average of diverse electrical energies. Millikan launched his investigation with a technique developed by the British-born physicist H. A. Wilson; it consisted essentially of measuring, first, the rate at which a charged cloud of water vapor fell under the influence of gravity and then the modified rate under the counterforce of an electric field. Using Stokes’s law of fall to determine the mass of the cloud, one could in principle compute the ionic charge. Millikan quickly recognized the numerous uncertainties in this technique, including the fact that evaporation at the surface of the cloud confused the measure of its rate of fall. Hoping to correct for this effect, he decided to study the evaporation history of the cloud while a strong electric Held held it in a stationary position.

But when Millikan switched on the powerful field, the cloud disappeared; in its place were a few charged water drops moving slowly in response to the imposed electrical force. He quickly realized that it would be a good deal more accurate to determine the electronic charge by working with a single drop than with the swarm of particles in a cloud. Finding that he could make measurements on water drops for up to forty-five seconds before they evaporated. Millikan arrived at a value for e in 1909 which he considered accurate to within 2 percent. More important, he observed that the charge on any given water drop was always an integral multiple of an irreducible value. This result provided the most persuasive evidence yet that electrons were fundamental particles of identical charge and mass.

Late in 1909 Millikan greatly improved the drop method by substituting oil for water. Because of the relatively low volatility of this liquid, he could measure the rise and fall of the drops for up to four and a half hours. Spraying the chamber with radium radiation, he could change the charge on a single drop at will. His overall results decisively confirmed the integral-multiple values of the total charge. As for the determination of e itself, Millikan found that Stokes’s law was inadequate for his experimental circumstances because the size of the drops was comparable with the mean free path of the air. Using the so-called Stokes-Cunningham version of the law, which took this condition into account, by late 1910 he had computed a charge for e of 4.891×10^-10 e.s.u. Realizing that the accuracy of this figure was no better than that of the key constants involved in the computation, Millikan painstakingly reevaluated the coefficient of viscosity of air and the mean-free-path term in the Stokes-Cunningham law. In 1913 he published the value for the electronic charge, 4.774±.009×10^-10 e.s.u., which would serve the world of science for a generation.

Off and on all the while, Millikan had continued his exploration of the photoelectric effect; about 1912, now aware of Einstein’s interpretation of it, he began an intensive experimental study of the phenomenon, with the aim of testing the formula relating the frequency of the incident light to the retarding potential which cut off the photocurrent. No experimentalist had yet succeeded in proving or disproving the validity of the equation. Millikan took great care to avoid the mistakes that he and other physicists had previously made. Since a spark source of ultraviolet light induced spurious voltages in the apparatus, he used a high-pressure mercury-quartz lamp arranged to suppress stray light, especially on the short wavelength side. To extend the range of test well into the visible region, he made targets of alkali metals which were photosensitive up to 6,000 Å. Where others had adulterated their results by using photosensitive materials as the reference for the cutoff voltage, Millikan employed a Faraday cage of welloxidized copper netting which was not photosensitive in the range of his incident radiation. Finally, he sought to reduce the inaccuracies introduced when the photocurrent near the cutoff point was too low to measure with precision. Having noticed that this current was highest when the metal was fresh, he fashioned his targets into thick cylinders and rigged up an electromagnetically operated knife to shave off the ends of the blocks.

By 1915, as the result of these meticulous investigations, Millikan had confirmed the validity of Einstein’s equation in every detail. He not only demonstrated the linear relationship between the cutoff potential and the frequency of the incident light but also showed that the intercept of the graphed data on the voltage axis equaled the contact electromotive force, or work potential, of the target metal, a quantity which he had measured independently, to within 0.5 percent. In addition Millikan proved that the slope of the line equaled the ratio of Planck’s constant to the electronic charge, and his work provided the best measure of h then available. Despite the conclusiveness of these results, Millikan did not believe that he had confirmed Einstein’s theory of light quanta but only his equation for the photoeffect. In the face of all the evidence for the wave nature of light, he was convinced, as were most other physicists of the day, that the equation had to be based on a false, albeit evidently quite fruitful, hypothesis.

By 1916, when Millikan completed his major work on the photoeffect, he had already assumed more than a mere professor’s role in the world of science. In 1913 he became a consultant to the research department of Western Electric, primarily to advise the company on vacuum tube problems. In 1914 he was elected to the American Philosophical Society and the American Academy of Arts and Sciences, in 1915 to the National Academy of Sciences, and in 1916 to the presidency of the American Physical Society, an office which he held for two years. Millikan also served as an associate editor of Physical Review from 1903 to 1916; and he was made an editor of Proceedings of the National Academy of Sciences …, which was started in the year of his election.

Early in 1917, after the United States broke diplomatic relations with Germany, Millikan went to Washington as a vice-chairman and director of research for the National Research Council, the organization which the National Academy of Sciences had recently created to help mobilize science for defense. Commissioned a lieutenant colonel in the Army Signal Corps, he served in his military capacity as the director of the Signal Corps Division of Science and Research and, in his National Research Council identity, as a member of the U.S. Navy’s Special Board on Antisubmarine Devices. After a brief postwar period back at Chicago, in 1921 Millikan accepted appointment as chairman of the executive council and director of the Norman Bridge Laboratory at the newly renamed California Institute of Technology in Pasadena. In effect the president of the school, he was an able fund raiser and its enthusiastic spokesman; and under his leadership it quickly developed into one of the most distinguished scientific centers in the world.

Managing all the while to supervise many doctoral and postdoctoral fellows, Millikan maintained an active research career throughout the interwar years. One of the important subjects he investigated was the ability of electric fields on the order of a few hundred thousand volts per centimeter to draw electrons out of cold metals. By 1926, working in collaboration with Carl F. Eyring, a Caltech graduate student, Millikan had completed a thorough study of the phenomenon, using tungsten wires threaded along the axis of a hollow cylinder in high vacuum. The two men found that the field current, to use the term they introduced, depended only on the field gradient, not on the potential difference, between the wire and the walls of the cylinder. More important, within wide limitations the current was also entirely independent of temperature. Pointing out that these results violated Owen W. Richardson’s theory of thermionic emission, Millikan and Eyring speculated that at relatively low temperatures some metallic electrons must not obey the law of equipartition. But in 1928 Oppenheimer, R. H. Fowler, and their co-workers showed independently that cold emission was a quantum mechanical result of the leakage of electrons through a potential barrier. In 1929 Charles C. Lauritsen, who was completing his doctoral research under Millikan, derived an empirical formula from their data which related the field current to the field gradient; and this equation was ultimately found to be experimentally indistinguishable from the quantum mechanical expression.

During the 1920’s Millikan also did significant research in the “hot spark” spectra. As he knew, a high potential difference would maintain a spark source of ultraviolet radiation across two electrodes in a vacuum. The relative ease with which such radiation was absorbed had made its study difficult. In 1915 Millikan proposed that one could get around the problem of absorption by enclosing the path between the spark and a photographic plate entirely in a vacuum. To maximize the intensity of the spectrum, he had a grating ruled that would throw most of the light into the first order. Shortly after the war, with the apparatus now working reliably, Millikan and Ira S. Bowen, another Caltech graduate student, embarked upon a thorough study of the ultraviolet spectra of the lighter elements up to copper. By 1924 they had found and identified some 1,000 new lines. They had also extended the observable spectrum down to 136.6 A and had helped to close the last gap between the optical and the X-ray frequencies.

In the course of this work, Millikan and Bowen found that the strongest lines were produced by atoms which had been stripped of their valence electrons. Since the spectra of such hydrogen-like atoms ought to contain multiplets, they began, about the end of 1923, to study the fine spectra in the ultraviolet. By early 1924 they had found that the 2s, 2p ₁ and 2p ₂ terms of the ultraviolet doublets corresponded precisely to the L_I L_II, and L_III levels associated with the X-ray spectra of the heavier elements. Moreover, exactly the same relationship existed between the M and N X-ray levels and the higher ultraviolet multiplel terms. Millikan and Bowen concluded that the X-ray doublet laws based on Sommerfeld’s relativistic orbital analysis could account for the doublets in the whole field of optics.

Yet, as they also pointed out in 1924, independently of Alfred Landé, this conclusion raised a serious difficulty for the theory of spectra. On the one hand, Sommerfeld’s relativistic analysis of the X-ray doublets assigned a different azimuthal quantum number to the L_II and L_III terms. On the other hand, Bohr’s spectral scheme accounted for the optical doublets by assuming different orientations for the same orbit; by definition, the p₁ and p₂ terms of the optical doublets possessed the same azimuthal quantum number. Since the results of Millikan and Bowen identified the L_II and L_III terms with the p₁ and p₂ levels, it seemed that one had to give up either Sommerfeld’s relativistic explanation or the Bohr scheme of spectra. Millikan and Bowen could find no way out of the dilemma; but their forceful statement of it in 1924, coupled with Landés, contributed to the ultimate resolution of the difficulty through G. E. Uhlenbeck and S. A. Goudsmit’s postulation of electron spin in 1925.

In the 1920’s Millikan also began an increasingly intensive program of research into the penetrating radiation which in mid-decade he would name “cosmic rays,” In 1912 the Austrian-born physicist Victor Hess had found that atmospheric ionization increased with altitude up to 12,000 feet. But although Hess had argued that some kind of radiation was coming from the heavens, most physicists still attributed the phenomenon to some terrestrial cause, such as electrical discharges from thunderstorms or radioactivity. Millikan’s initial experiments in the field, done with an unmanned sounding balloon in 1922 to a height of fifteen kilometers and with lead-shielded electroscopes atop Pike’s Peak in 1923, failed to decide in favor of either interpretation. In the summer of 1925 Millikan proposed to settle the question by measuring the variation of ionization with depth in Muir Lake and Lake Arrowhead in the mountains of California. Snow-fed and separated by many miles, as well as 6,675 feet of atmosphere, each was likely to be free of both local radioactive disturbances and whatever atmospheric peculiarities might affect the ionization in the other.

Millikan’s electroscopie measurements showed that the intensity of ionization at any given depth in Lake Arrowhead was the same as the intensity six feet lower in Muir Lake. Since the layer of atmosphere between the surfaces of the two lakes had precisely the absorptive power of six feet of water, the results decisively confirmed that the radiation was coming from the cosmos. Moreover, since the intensity of the ionization showed no diurnal variation, the radiation was uniformly distributed over all directions in space. And, finally, since Millikan detected ionization as far below the top of the atmosphere as the combined air and water equivalent of six feet of lead, it was evident that cosmic rays were a good deal more energetic than even the hardest known gamma rays.

To penetrate six feet of lead, charged particles would have to possess stores of energy then considered impossibly large; accordingly, Millikan assumed that cosmic rays must consist of photons. In 1926 he tested this assumption experimentally with what he considered confirmatory results. If cosmic rays were charged particles, their trajectories would be affected by the earth’s magnetic field, so that more of them would strike the earth at higher than at lower latitudes. But Millikan could detect virtually no difference in cosmic ray flux at Lake Titieaea in South America from that at Muir Lake. And, although he ran his electroscope while sailing back from Mollendo, Peru, to Los Angeles, he found no variation of intensity with latitude at sea level.

Employing the photonic interpretation of cosmic rays, Millikan developed a theory of their origin in 1928. Combining the data from the balloon flight of 1922 with that of his terrestrial surveys, he graphed a curve of ionization versus depth which covered the range from sea level up to virtually the top of the atmosphere. Because no single coefficient of absorption could account for the curve, he inferred that cosmic rays were spread across a spectrum of energies. Going further, he argued that the experimental curve could be constructed from three different curves, each representing a different coefficient of absorption. According to this analysis, cosmic ray energies were not generally distributed but were clustered in three distinct bands.

To account for these bands, Millikan introduced what he called the “atom-building hypothesis.” Using Dirac’s formula for absorption through Compton scattering, Millikan computed the energy of the three bands from their absorption coefficients and found them equal to 26, 110, and 220 MEV. These figures equaled the mass defects of hydrogen, oxygen, and silicon, which were known to be three of the most abundant elements of the universe. Millikan concluded that the photons striking the earth must be produced when four atoms of hydrogen somehow fused to form helium, sixteen to form oxygen, and twenty-eight to form silicon. In his summary of the argument, cosmic rays were the “birth cries” of atoms, a phrase which quickly achieved a good deal of notoriety among both the scientific and the lay publics.

Although in the late 1920’s most physicists agreed with Millikan that cosmic rays were photons, few accepted his atom-building hypothesis. He had no proof of the uniqueness of his three absorption coefficients and could not convincingly explain away the kinetic difficulties involved in the spontaneous union of sixteen hydrogen atoms into oxygen, let alone twenty-eight into silicon. Moreover, some of his own experimental evidence cast doubt on the validity of using the Dirac formula to compute cosmic ray energies. Then, at the beginning of the 1930s, Millikan’s assumption that the primary radiation consisted of photons was refuted by the work of other experimentalists, especially by Arthur Compton’s conclusive detection of a latitude effect in 1932.

Millikan hotly contested Compton’s findings. He had repeated his search for a latitude effect in the late 1920’s, and in late 1932 he did so once more, again without success. But Millikan was the victim of experimental circumstance. In the longitudinal region of California, the dip in cosmic ray intensity began quite suddenly in the neighborhood of Los Angeles and quickly reached its maximum fall of some 7 percent less than two days’ sail south of the city. In Millikan’s initial search for the latitude effect—the voyage from Mollendo, Peru, to Los Angeles—his estimated error had been 6 percent. In most of his later searches, he went to the north of Pasadena, where the rise in intensity was too small to detect easily. In 1932 he sent H. Victor Neher, a young collaborator at Calteeh, on a voyage to the south; but Neher did not get his electroscope working before he had passed the region of the dip.

By 1933, with Neher having now found a latitude effect, Millikan had admitted that some percentage of cosmic radiation must consist of charged particles. By 1935 he had also rejected the atom-building hypothesis, mainly because it was now clear that the bulk of cosmic radiation possessed energies much higher than the mass defects of the abundant elements. All the same, despite a vast array of contrary evidence and the overwhelming body of professional opinion, Millikan clung tenaciously to the assumption that some fraction of the primary cosmic radiation could be photons. in the late 1930’s and early 1940’s he searched for evidence in support of this view, measuring cosmic ray intensities around the world at sea level, in airplanes at high altitudes, and with unmanned sounding balloons up to the top of the atmosphere. On the basis of this data, he also developed a theory that cosmic ray photons originated in the spontaneous annihilation of atoms in interstellar space. No more convincing than its predecessor, this hypothesis became completely untenable, as Millikan himself admitted a few years before his death, after the detection of the π-meson in 1947 made it clear that the primary cosmic radiation consisted almost entirely of protons.

But however wrongheaded Millikan had been, his cosmic ray research yielded a valuable fund of experimental data. Moreover, in 1934, independently of Jacob Clay, he detected the variation of the latitude effect with longitude because of the dissymmetry of the earth’s magnetic field. In a roundabout way even the atom-building hypothesis strikingly benefited the progress of science. In the late 1920’s, troubled by the discrepancy between his experimental data and the predictions of both the Dirac absorption formula and its successor, the Klein-Nishina formula, Millikan recognized that he needed a measure of cosmic ray energies that was not based on absorption coefficients. To obtain a direct determination, he put Carl Anderson, a young research fellow at Caltech, to work with a cloud chamber set in a powerful magnetic field. In 1931 Anderson’s studies of the trajectories of charged particles showed conclusively that the absorption of cosmic rays resulted from nuclear encounters as well as from Compton scattering. They also led to his detection of the positron in 1932.

Between the wars Millikan played a prominent role in the affairs of his profession. The president of the American Association for the Advancement of Science in 1929 and the holder of various offices in the National Academy of Sciences and the National Research Council, he was especially active as a member of the NRC fellowship board and as foreign secretary of the Academy. From 1922 to 1932 Millikan served as the American representative to the Committee on Intellectual Cooperation of the League of Nations. Throughout the interwar period he participated in the International Research Council; its successor, the International Council of Scientific Unions; and the affiliate of both, the International Union of Pure and Applied Physics. In 1933 Millikan was appointed by President Franklin D. Roosevelt to the Science Advisory Board, a joint venture of the Academy and the federal government to find ways to use science for economic recovery.

Millikan was an able popularizer and lecturer, and after he won the Nobel Prize in 1923 he became perhaps the most famous American scientist of his day. An outspoken religious modernist, he was a leading exponent of the reconcilability of science and religion in the l920’s, the decade of the Scopes trial. Politically, Millikan was a conservative Republican. During the 1930’s he vigorously opposed the New Deal, repeatedly denounced governmental intervention in the economy, and argued that the promotion of science, because it led to new industries and new jobs, was a much sounder way to achieve economic recovery. Always an internationalist, Millikan believed firmly in collective security. In the late 1930’s, unlike many conservative Republicans at the time, including his good friend Herbert Hoover, he helped propagandize in favor of aid to the Allies; by early 1941 he was encouraging the conversion of Caltech from academic to military purposes.

During the war Millikan turned over an increasing fraction of his administrative responsibilities at Caltech to the younger staff members who were running the various defense projects. In 1946 he retired from his professorship and the chairmanship of the executive council. He remained active as a public lecturer and spoke frequently on the subject of science and religion. He was cool to the creation of the National Science Foundation and spoke often against federal aid to education. By the time of his death, Millikan had been awarded numerous medals, even more honorary degrees, and membership in twenty-one foreign scientific societies, including the Royal Society of London and the Institut de France.

BIBLIOGRAPHY

I. Original Works. A complete bibliography of Millikan’s published work, which includes close to 300 scientific papers, is in Lee A. DuBridge and Paul S. Epstein, “Robert Andrews Millikan,” in Biographical Memoirs. National Academy of Sciences, 33 (1959), 241–282. In The Autobiography of Robert A. Millikan (New York, 1950) Millikan provided valuable accounts of his childhood and education, work on the electronic charge and the photoeffect, and involvement in the mobilization of science during World War I; curiously, he devoted little space to his research in hot spark spectra or cosmic rays, and his account of the development of the California Institute of Technology must be used with special care. Millikan left a voluminous body of correspondence, which is now in the Caltech archives. Dating in the main from 1921, the collection contains substantial materials on the National Academy of Sciences-National Research Council and the administration of the California Institute of Technology, as well as a sizable amount of family and scientific letters. Another important batch of Millikan’s letters is in the papers of George Ellery Hale in the Caltech archives, which were also published in a microfilm edition (Pasadena, 1968) under the editorship of Daniel J. Kevles. The locations of other letters to and from Millikan are given in Thomas S. Kuhn, et al., Sources for the History of Quantum Mechanics (Philadelphia, 1967), 68.

II. Secondary Literature. Paul S. Epstein wrote an excellent résumé of Milikan’s scientific work in “Robert A. Millikan as Physicist and Teacher,” in Reviews of Modern Physics, 20 (Jan. 1948), 10–25, a volume published in honor of Millikan’s eightieth birthday. A condensed version of Epstein’s essay occupies part of the memoir written with DuBridge (see above), which is on the whole a useful introduction to Millikan’s life. Millikan the famous scientist is treated in Daniel J. Kevles, “Millikan: Spokesman for Science in the Twenties,” in Engineering and Science, 32 (Apr. 1969), 17–22.

Daniel J. Kevles